Originally Posted by Seaver

The numbers are the thickness of the oil. In the oil commercials you hear about "viscosity breakdown", basically the oil looks it's stickyness factor. Thinner oils work better in cold environments, as it quickly circulates through the engine at startup to protect the metal-on-metal contacts. Thicker oils tend to take a bit longer (a minute or so) in very cold weather to warm up and go to the engine. That means, however, you're left momentarily with little oil in your engine. Once it's up and running, however, you'll be fine.

Thicker oils (10w40, 20w40) are much better at protecting engines over the long term and in hotter environments. The thicker oil means its viscosity will adhear better to the metal parts and stick more in the minute defection in the metals which provides more lubrication.

Personally I stick to the heavier stuff. Then again I live in the Texas heat, with a truck with a large V8, and have over 140k miles. So the lighter oils simply wouldn't hold up.

5w20 for you sounds extremely lightweight. While it may be in your owners manual, it just seems to me unless your engine is powered by a lawnmower engine you'll want something thicker. At next oil change I'd go with the 10w30 or 40. 10 weight is good for cars and works well in cold weather.